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The relationship between old park trees and saproxylic beetles

Mature and veteran trees (often pollarded) are very much important, in terms of the ecological value they provide. An array of organisms utilise such trees, ranging from bats to birds, and from beetles to moths. However, when these trees are found within amenity parklands, the presence of visitors means they will be managed both for their safety and for their amenity value. Therefore, deadwood may be removed, and entire specimens may be felled because of the risk associated with their retention. These actions have adverse impacts upon constituent species, with saproxylic beetles potentially suffering quite notably. In spite of this, little research has been completed on old parkland trees and the saproxylic beetle species they support.

In addition, because ‘open’ old trees are declining in abundance, both because of their removal and due to the reversion of some old pastures and parklands to higher woodland (mainly due to a reduction in grazing, though woodlands may also be created), the viable habitat for many beetles is being lost. Old and open trees may thus be found commonly only in large parks that are popular attractions, or within private estates where maintenance costs are less of a concern (and there is a very low risk of a failing tree causing harm to a person). Of course, active wood pastures also contain many old trees, and some of these are (or were) deer parks.

In this study, undertaken within a 150km radius of Stockholm, Sweden (see map below), the author looks solely at limes (Tilia spp.), which are commonly found scattered within parks (dominated by oaks) as mature or veteran specimens – often, but not always, as pollards (up until around 1900-1950, when such pollarding lapsed completely). In the natural stands, Tilia cordata is most common, though in parks the prevalent species of lime is Tilia x europaea. Across the sites surveyed, of which eight were designated ‘open’, eleven ‘regrown’, and eight ‘park’, four limes (at each location) that had the potential (through the presence of hollows) to act as a host to beetles were surveyed. Accompanied by the sampling of beetle species (through the use of window traps, active from May through to August / September in the survey year), data relating to the size (circumference) of the limes, the total number of hollow limes on the site, and also the location of the trees was captured.

The author thus poses two questions that are to be answered, and these are: (1) will park trees have as much beetle diversity as natural stands?, and; (2) is there variation in natural stands, in relation to whether the site is grazed (open) or regenerating (regrown)?

For more detailed descriptions of each site, please see Table 6 in the report (linked at the bottom).

At a slight tangent before moving on to the results, typically we may associate saproxylic beetles with species such as oak (Quercus spp.), though it is remarked that beetles won’t always have a host preference in terms of the tree species, but in terms of the qualities of the tree itself (hollows, cavities, snags, dead branches, and so on). Granted, we can observe broadleaved obligates, coniferous obligates, and generalists.

Results

Following on from the trapping period, 14,460 individuals of 323 different beetle species were found. Around 10% of the total number of individuals found were red-listed species, though their presence accounted for 15% of the total species observed. Most of the species found were associated with the wood and bark of their host, and very few were associated with sap runs. The below table outlines such results.

Here we can see the breakdown of surveyed individuals and species, across all sites combined.

In terms of which sites had the greatest diversity of species, it was found that ‘open’ sites always ranked the highest (see the graphs below). However, the differences were not always significant. Furthermore, the more lime trees found on a site, the lower the diversity of beetle species found in wood and bark. The circumference of a lime tree was also not considered to be important in determining how abundant populations would be within the host, asides from for red-listed species found only in hollows.

A number of graphs that compare the different locations against the number of species present, according to specific data sets.

Quite importantly, there was generally no significant difference in species abundance and diversity between natural and park sites, and when park sites are compared to sites regenerating they are shown to support a greater diversity of beetle species in certain instances (such as for red-listed species found in hollows). The fact that park limes were found to support a lesser array of beetle species for wood and bark species (for those not red-listed), the author remarks, is because many of the trees in the parks are managed (and thus, any deadwood may be removed).

Additionally, whilst the composition of species between park and natural sites was found to be significantly different, many species were found using both parks and natural sites (only sixteen species displayed exclusive preference to parks or natural stands). This means that a park has the potential to host a diversity of saproxylic beetle species similar to that of a natural site. Many species were also found to prefer more sun-exposed conditions, and the author notes that where a park tree may be lacking in deadwood a sun-exposed setting may compensate for the loss of direct habitat associated with deadwood presence. This may perhaps explain why more red-listed species were found in parks, when compared to regrown sites – they require more ‘ideal’ hosts, compared to generalist species. Furthermore, southern sites were found to possess more beetle species, and this is considered to be due to the more favourable climatic conditions (generally, they are warmer).

As touched upon earlier, trunk circumference was not found to be a significant factor in determining species diversity. This conflicts with many previous studies, though the fact that most of the limes surveyed were classed as ‘ancient’ may explain this lack in significance – they were already very large in circumference. If limes of a ‘younger’ age (perhaps ‘only’ veterans) were surveyed, trunk circumference may have been significant (or at least more influential).

In light of all of the above, we can observe that parks may very well be important for saproxylic beetles – at times, more so than natural stands where a woodland is regenerating around old individuals. This means that the suppression of regrowth may be beneficial for saproxylic beetles, though at the same time there must be an awareness of needing to recruit new veteran and ancient trees. All trees have a point at which they die, and by simply retaining existing old trees and not identifying and safeguarding future ones, beetle species will decline and disappear when the current population of old trees drops or ceases to be.

It may also be wise to retain deadwood removed from park trees, either at the base of the tree or in a large pile somewhere else on the site. This may be crucial if beetles exist within the wood structure itself, as its retention allows for the larvae to complete their life cycle by emerging as adults from the wood and (hopefully) reproducing. Not only this, but active management of existing trees may also be beneficial. Many old pollards may be considered highly hazardous and at risk of collapsing, and as pollarding will slow down the rate of growth, viable habitats (such as hollows) can be retained for longer periods in more suitable conditions if the trees do undergo new cycles of pollarding (or crown reduction). If done properly, this may also enable for new trees to more effectively succeed (and eventually replace) the existing old tree population.

Most importantly, old park trees should not be felled unless absolutely necessary. Old trees are crucial in terms of the saproxylic species they support (of which some are red-listed), and park managers should therefore incorporate such ecological conservation measures into the management plan for the site as a whole.